Radiation filter, spectrometer and imager using a micro-mirror array

ABSTRACT

A spectrometer (10) includes a two-dimensional array of modulatable micro-mirrors (18), a detector (20), and an analyzer (22). The array of micro-mirrors is positioned for receiving individual radiation components forming a part of an input radiation source. The micro-mirrors are modulated at different modulation rates in order to reflect individual radiation components therefrom at known and different modulation rates. The micro-mirror array combines a number of the reflected individual radiation components and reflects the combined components to the detector. The detector is oriented to receive the combined radiation components reflected from the array and is operable to create an output signal representative thereof. The analyzer is operably coupled with the detector to receive the output signal and to analyze at least some of the individual radiation components making up the combined reflection. By using a micro-mirror that receives individual radiation components and then modulates the radiation components at different rates, all of the radiation components can be focused onto a single detector to maximize the signal-to-noise ratio of the detector. A variable band pass filter spectrometer, variable band reject filter spectrometer, variable multiple band pass filter spectrometer, and a variable multiple band reject filter spectrometer utilizing the same invention are also disclosed.

RELATED APPLICATION

This is a continuation application of Ser. No. 09/289,482 filed Apr. 9,1999, now U.S. Pat. No. 6,046,808.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to imagers, spectrometers, variable bandpass filters or variable multiple band pass filters that include amicro-mirror array having a plurality of micro-mirrors that each can beindividually modulated at a different modulation rate.

2. Description of the Prior Art

Imagers employ either a two-dimensional (2D) multichannel detector arrayor a single element detector. Imagers using a 2D detector array measurethe intensity distribution of all spatial resolution elementssimultaneously during the entire period of data acquisition. Imagersusing a single detector require that the individual spatial resolutionelements be measured consecutively via a raster scan so that each one isobserved for a small fraction of the period of data acquisition. Priorart imagers using a plurality of detectors at the image plane canexhibit serious signal-to-noise ratio problems. Prior art imagers usinga single element detector can exhibit more serious signal-to-noise ratioproblems. Signal-to-noise ratio problems limit the utility of imagersapplied to chemical imaging applications where subtle differencesbetween a sample's constituents become important.

Spectrometers are commonly used to analyze the chemical composition ofsamples by determining the absorption or attenuation of certainwavelengths of electromagnetic radiation by the sample or samples.Because it is typically necessary to analyze the absorptioncharacteristics of more than one wavelength of radiation to identify acompound, and because each wavelength must be separately detected todistinguish the wavelengths, prior art spectrometers utilize a pluralityof detectors, have a moving grating, or use a set of filter elements.However, the use of a plurality of detectors or the use of a macromoving grating has signal-to-noise limitations. The signal-to-noiseratio largely dictates the ability of the spectrometer to analyze withaccuracy all of the constituents of a sample, especially when some ofthe constituents of the sample account for an extremely small proportionof the sample. There is, therefore, a need for imagers and spectrometerswith improved signal-to-noise ratios.

Prior art variable band pass filter spectrometers, variable band rejectfilter spectrometers, variable multiple band pass filter spectrometersor variable multiple band reject filter spectrometers typically employ amultitude of filters that require macro moving parts or other physicalmanipulation in order to switch between individual filter elements orsets of filter elements for each measurement. Each filter elementemployed can be very expensive, difficult to manufacture and all arepermanently set at the time of manufacture in the wavelengths (bands) ofradiation that they pass or reject. Physical human handling of thefilter elements can damage them and it is time consuming to changefilter elements. There is, therefore, a need for variable band passfilter spectrometers, variable band reject filter spectrometers,variable multiple band pass filter spectrometers or variable multipleband reject filter spectrometers without a requirement for discrete(individual) filter elements that have permanently set band pass or bandreject properties. There is also a need for variable band pass filterspectrometers, variable band reject filter spectrometers, variablemultiple band pass filter spectrometers or variable multiple band rejectfilter spectrometers to be able to change the filters corresponding tothe bands of radiation that are passed or rejected rapidly, withoutmacro moving parts and without human interaction.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention solves the above-described problems and provides adistinct advance in the art by providing an imager or spectrometer thatis much less sensitive to ambient noise and that can effectively operateeven when used in environments with a high level of ambient radiation.The invention further advances the art of variable band pass filterspectrometers, variable band reject filter spectrometers, variablemultiple band pass filter spectrometers or variable multiple band rejectfilter spectrometers by providing a variable band pass filterspectrometer, variable band reject filter spectrometer, variablemultiple band pass filter spectrometer or variable multiple band rejectfilter spectrometer that:

(1) does not require the selection of the bands of wavelengths passed orrejected at the time of manufacture;

(2) allows the selection of any desired combination of bands ofwavelengths that are passed or rejected;

(3) reduces the time to change the bands of wavelengths passed orrejected; and

(4) requires no macro moving parts to accomplish a change in the bandsof wavelengths passed or rejected.

The present invention broadly includes a two-dimensional array ofmodulateable micro-mirrors, a detector, and an analyzer. The array ofmicro-mirrors is positioned for receiving an image. The micro-mirrorsare modulated at known and selectively different modulation rates inorder to reflect individual spatially distributed radiation componentsof the image therefrom at known and different modulation rates towardthe detector.

The detector is oriented to receive the combined radiation componentsreflected from the array and is operable to generate an output signalrepresentative of the combined radiation incident thereon. The analyzeris operably coupled with the detector to receive the output signal andto demodulate the signal to recover signals representative of each ofthe individual spatially distributed radiation components of the image.The analyzer can be configured to recover all reflected components or toreject some unnecessary components of the recovered signals from thecombined reflections.

By using a micro-mirror that receives the individual spectral or spatialradiation components and then modulates the radiation components atdifferent modulation rates, all of the radiation components can befocused onto a single detector and then later demodulated to maximizethe signal-to-noise ratio of the detector.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A preferred embodiment of the present invention is described in detailbelow with reference to the attached drawing figures, wherein:

FIG. 1 is a schematic diagram illustrating a spectrometer constructed inaccordance with one embodiment of the invention;

FIG. 2 is a plan view of a micro-mirror array used in the presentinvention;

FIG. 3 is a schematic diagram of two micro-mirrors illustrating themodulations of the mirrors of the micro-mirror device of FIG. 2;

FIG. 4 is a graph illustrating an output signal of the spectrometer whenused to analyze the composition of a sample;

FIG. 5 is a graph illustrating an output signal of the imager when usedfor imaging purposes;

FIG. 6 is a schematic diagram illustrating an imager constructed inaccordance with a preferred embodiment of the invention;

FIG. 7 is an illustration of the input to the DMA Filter Spectrometerand its use to pass or reject wavelength of radiation specific toconstituents in a sample.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawing figures and particularly FIG. 1, aspectrometer assembly 10 constructed in accordance with one embodimentof the invention is illustrated. The spectrometer broadly includes asource 12 of electromagnetic radiation, a mirror and slit assembly 14, awavelength dispersing device 16, a spatial light modulator 18, adetector 20, and an analyzing device 22.

In more detail, the electromagnetic radiation source 12 is operable toproject rays of radiation onto or through a sample 24 such as a sampleof body tissue or blood. The radiation source may be any device thatgenerates electromagnetic radiation in a known wavelength spectrum suchas a globar, hot wire, or light bulb that produces radiation in theinfrared spectrum. To increase the amount of rays that are directed tothe sample, a parabolic reflector 26 may be interposed between thesource 12 and the sample 24.

The mirror and slit assembly 14 is positioned to receive the radiationrays from the source 12 after they have passed through the sample 24 andis operable to focus the radiation onto and through the entrance slit30. The collection mirror 28 focuses the radiation rays through slit 30and illuminates the wavelength dispersing device 16.

The wavelength dispersing device 16 receives the beams of radiation fromthe mirror and slit assembly 14 and disperses the radiation into aseries of lines of radiation each corresponding to a particularwavelength of the radiation spectrum. The preferred wavelengthdispersing device is a concave diffraction grating; however, otherwavelength dispersing devices such as a prism may be utilized.

The spatial light modulator 18 receives radiation from the wavelengthdispersing device 16, individually modulates each spectral line, andreflects the modulated lines of radiation onto the detector 20. As bestillustrated in FIG. 2, the spatial light modulator is preferably amicro-mirror array that includes a semi-conductor chip or piezioelectric device 32 having an array of small reflecting surfaces 34thereon that act as mirrors. One such micro-mirror array is manufacturedby Texas Instruments and is described in more detail in U.S. Pat. No.5,061,049, hereby incorporated into the present application byreference. Those skilled in the art will appreciate that other spatiallight modulators such as a magneto optic modulator or a liquid crystaldevice may be used instead of the micro-mirror array.

The semi-conductor 32 of the micro-mirror array 18 is operable toindividually tilt each mirror along its diagonal between a firstposition depicted by the letter A and a second position depicted by theletter B in FIG. 3. In preferred forms, the semi-conductor tilts eachmirror 10 degrees in each direction from the horizontal. The tilting ofthe mirrors 34 is preferably controlled by the analyzing device 22,which may communicate with the micro-mirror array 18 through aninterface 37.

The micro-mirror array 18 is positioned so that the wavelengthdispersing device 16 reflects each of the lines of radiation upon aseparate column or row of the array. Each column or row of mirrors isthen tilted or wobbled at a specific and separate modulation frequency.For example, the first row of mirrors may be wobbled at a modulationfrequency of 100 Hz, the second row at 200 Hz, the third row at 300 Hz,etc.

The mirrors are calibrated and positioned so that they reflect all ofthe modulated lines of radiation onto the detector 20. Thus, even thougheach column or row of mirrors modulates its corresponding line ofradiation at a different modulation frequency, all of the lines ofradiation are focused onto a single detector.

The detector 20, which may be any conventional radiation transducer orsimilar device, is oriented to receive the combined modulated lines ofradiation from the micro-mirror array 18. The detector is operable forconverting the radiation signals into a digital output signal that isrepresentative of the combined radiation lines that are reflected fromthe micro-mirror array. A reflector 36 may be interposed between the asmicro-mirror array 18 and the detector 20 to receive the combinedmodulated lines of radiation from the array and to focus the reflectedlines onto the detector.

The analyzing device 22 is operably coupled with the detector 20 and isoperable to receive and analyze the digital output signal from thedetector. The analyzing device uses digital processing techniques todemodulate the signal into separate signals each representative of aseparate line of radiation reflected from the micro-mirror array. Forexample, the analyzing device may use discrete Fourier transformprocessing to demodulate the signal to determine, in real time, theintensity of each line of radiation reflected onto the detector. Thus,even though all of the lines of radiation from the micro-mirror arrayare focused onto a single detector, the analyzing device can separatelyanalyze the characteristics of each line of radiation for use inanalyzing the composition of the sample.

The analyzing device is preferably a computer that includes spectralanalysis software. FIG. 4 illustrates an output signal generated by theanalyzing device. The output signal is a plot of the absorptioncharacteristics of five wavelengths of radiation from a radiation sourcethat has passed through a sample.

In a preferred embodiment of the invention illustrated in FIG. 6, theinvention is used for digital imaging purposes. As an imaging device, animage of a sample 38 is focused onto a micro-mirror array 40 and eachmicro-mirror in the array is modulated at a different modulation rate.The micro-mirror array geometry is such that some or all of thereflected radiation impinges upon a single detector element 42 and issubsequently demodulated to reconstruct the original image improving thesignal-to-noise ratio of the imager. Specifically, an analyzing device44 digitally processes the combined signal to analyze the magnitude ofeach individual pixel. FIG. 5 is a plot of a three dimensional imageshowing the magnitude of each individual pixel.

FIG. 7 illustrates the output of a digital micro-mirror array (DMA)filter spectrometer used as a variable band pass filter spectrometer,variable band reject filter spectrometer, variable multiple band passfilter spectrometer or variable multiple band reject filterspectrometer. In this example the combined measurement of theelectromagnetic energy absorbed by sample constituents A and C is ofinterest. The shaded regions in FIG. 7 illustrate the different regionsof the electromagnetic spectrum that will be allowed to pass to thedetector by the DMA filter spectrometer. The wavelengths ofelectromagnetic radiation selected to pass to the detector correspond tothe absorption band for compound A and absorption band for compound C ina sample consisting of compounds A, B, and C. The spectral regioncorresponding to the absorption band of compound B and all otherwavelengths of electromagnetic radiation are rejected. Those skilled inthe art will appreciate that the DMA filter spectrometer is not limitedto the above example and can be used to pass or reject any combinationof spectral resolution elements available to the DMA.

As a DMA filter imager the spatial resolution elements (pixels) of animage can be selectively passed or rejected (filtered) according to therequirements of the image measurement. The advantages of both the DMAfilter spectrometer and DMA filter imager are:

(1) All spectral resolution elements or spatial resolution elementscorresponding to the compounds of interest in a particular sample can bedirected simultaneously to the detector for measurement. This has theeffect of increasing the signal-to-noise ratio of the measurement.

(2) The amount of data requiring processing is reduced. This reducesstorage requirements and processing times.

Although the invention has been described with reference to thepreferred embodiment illustrated in the attached drawing figures, it isnoted that equivalents may be employed and substitutions made hereinwithout departing from the scope of the invention as recited in theclaims. For example, although the imager and spectrometer areparticularly useful for spectral analysis in the infrared spectrum, theycan be used for analysis in any wavelength spectrum.

It can be noted that in FIG. 6 the sample is illuminated with broadbandradiation and the image of the broadly illuminated sample is focusedonto the DMA for encoding before being passed to a detector or DMAtunable filter spectrometer. This configuration we will call post-sampleencoding. In another configuration we will call pre-sample encoding thebroadband source illuminates the DMA and the modulations of themicro-mirrors in the DMA encode the source radiation prior to impingingupon the sample. The reflected radiation is then collected from thesample and directed onto the detector of into the DMA tunable filterspectrometer for spectral characterization.

Having thus described the preferred embodiment of the invention, what isclaimed as new and desired to be protected by letters patent includesthe following:

I claim:
 1. An analysis assembly for analyzing the absorption orattenuation by a sample of input radiation having a plurality ofindividual radiation components, said assembly comprising:a plurality ofmodulatable micro-mirrors located for receiving individual radiationcomponents forming a part of said input radiation, said micro-mirrorsbeing modulatable at different modulation rates in order to reflectindividual radiation components therefrom at known and differentmodulation rates and for combining a number of the reflected individualradiation components to generate a combined reflection; a detectororiented to receive said combined reflection from said micro-mirrors,and operable to create an output signal representative of said combinednumber of individual radiation components; and an analyzer operablycoupled with said detector to receive said output signal, and providingan individual analyses of at least some of said individual radiationcomponents making up said combined reflection.
 2. The assembly of claim1, said analyzer comprising a computer operably coupled with saiddetector and said micro-mirrors for controlling the modulation of saidmicro-mirrors and for providing said individual analyses.
 3. Theassembly of claim 1, further including a reflector positioned betweenthe detector and the micro-mirrors and operable to receive said combinedreflection from said micro-mirrors and to focus the combined reflectionon said detector.
 4. The assembly of claim 1, said analyzer operable toperform a Fourier analysis upon said output.
 5. The assembly of claim 1,said individual analyses comprising the energy magnitude of saidindividual radiation components.
 6. The assembly of claim 1, said inputradiation being electromagnetic radiation.
 7. The assembly of claim 1,said individual radiation components comprising different, substantiallyconstant wavelength radiation components.
 8. The assembly of claim 1,said individual radiation components comprising spatial resolutioncomponents derived from an image.
 9. Apparatus comprising:an assemblyoperable to generate input radiation having a plurality of individualradiation components; a plurality of modulatable micro-mirrors locatedfor receiving individual radiation components forming a part of saidinput radiation, said micro-mirrors being modulatable at differentmodulation rates in order to reflect individual radiation componentstherefrom at known and different modulation rates and with micro-mirrororientations for combining a number of the reflected individualradiation components to generate a combined reflection; a detectororiented to receive said combined reflection from said micro-mirrors,and operable to create an output signal representative of said combinednumber of individual radiation components; and an analyzer operablycoupled with said detector to receive said output signal, and providingan individual analyses of at least some of said individual radiationcomponents making up said combined reflection.
 10. The assembly of claim9, said analyzer comprising a computer operably coupled with saiddetector and micro-mirrors for controlling the modulation of saidmicro-mirrors and for providing said individual analyses.
 11. Theassembly of claim 9, further including a reflector positioned betweenthe detector and the micro-mirrors and operable to receive said combinedreflection from said micro-mirrors and to focus the combined reflectionon said detector.
 12. The assembly of claim 9, said analyzer operable toperform a mathematical manipulation upon said output.
 13. The assemblyof claim 9, said individual analyses comprising the energy magnitude ofsaid individual radiation components.
 14. The assembly of claim 9, saidinput radiation being infrared radiation.
 15. The assembly of claim 9,said individual radiation components comprising different, substantiallyconstant wavelength radiation components.
 16. The assembly of claim 9,said individual radiation components comprising spatial resolutioncomponents derived from an image.
 17. An analysis assembly for analyzingthe absorption or attenuation by a sample of input radiation having aplurality of individual radiation components, said assembly comprising:aplurality of modulatable devices located for receiving individualradiation components forming a part of said input radiation, saiddevices being modulatable at different modulation rates in order toproject individual radiation components therefrom at known and differentmodulation rates for combining a number of the projected individualradiation components to generate a combined projection; a detectororiented to receive said combined projection from said devices, andoperable to create an output representative of said combined number ofindividual radiation components; and an analyzer operably coupled withsaid detector to receive said output, and providing an individualanalyses of at least some of said individual radiation components makingup said combined projection.
 18. The analysis assembly of claim 17,wherein said assembly is operable for use as a variable filter.